The present invention relates generally to television signal transmission and receiving systems and, more particularly, to a system wherein the synchronization components of a conventional NTSC television signal are displaced in time rendering the resulting format particularly useful in subscription and multi-channel cable television applications.
In subscription television systems television programming signals are typically transmitted, either "over-the-air" or through a suitable cable network, in a scrambled form rendering the broadcast video information largely unviewable when received by a conventional television receiver. In order to supply an unscrambled video display to the system subscribers, each subscriber is provided with a suitable decoder operable for unscrambling the broadcast signals and for coupling the unscrambled signals to a conventional television receiver for viewing. Security is, of course, a prime consideration in the design of such systems. That is, the scrambling technique employed should be of a nature making the unauthorized decoding of the broadcasts extremely difficult in order to deter the would-be pirate. At the same time, the scrambling technique must accommodate reliable decoding by authorized system subscribers.
In this connection, subscription television systems typically employ one or both of two general techniques for scrambling the video information of a broadcast television signal. One of these techniques involves the inversion of the video signal occuring during selected horizontal scanning lines of the broadcast television signal and the subsequent re-inversion thereof by a suitable decoder or unscrambler connected to a television receiver. The other scrambling technique in widespread use is that of sync suppression. Suppression of the horizontal and/or vertical synchronization components of a broadcast television signal below the average video level will cause the deflection circuits of a normal television receiver to behave erratically such that a scrambled video image is produced on the display screen of the receiver. In addition, the ability of the television receiver to use the color reference burst associated with the horizontal synchronization signals is severely degraded thereby causing inaccurate color reproduction. Unfortunately, most currently known systems employing either video inversion or sync suppression scrambling techniques are relatively insecure in that unauthorized or pirated decoders keyed to unscramble the video broadcasts are relatively easy to design and manufacture.
It is therefore a basic object of the present invention to provide an improved scrambling technique for use in a television transmission and receiving system. It is a more specific object of the invention to provide a subscription television transmission and receiving system using a highly secure scrambling technique adapted for detering the unauthorized decoding of scrambled video broadcasts. These and other objects are accomplished according to the invention by interchanging the positions of a video signal patch and the horizontal synchronization component of a standard NTSC television signal to produce a time displaced horizontal synchronization signal. Alternatively, or in combination therewith, the standard vertical synchronization component is replaced with a relatively wide time displaced vertical synchronization pulse with the remainder of the associated vertical interval being maintained at a white level. The standard horizontal and vertical synchronization components are restored by suitable decoders to enable an authorized subscriber to unscramble the broadcast programs for viewing.